1. Field of the Invention
This invention relates to an improved base film for a magnetic recording tape, which is well-balanced in mechanical properties as measured along both the longitudinal and transverse directions. This base film exhibits a specified F-5 value as measured along both the longitudinal and transverse directions and a specified ratio of the average surface roughness to the average particle size of particles included therein. By the term "F-5 value" used herein is meant a tensile strength at an elongation of 5%.
2. Description of the Prior Art
A film composed mainly of a polyester, especially polyethylene terephthalate (hereinafter, for brevity, referred to as "PET"), which has been biaxially drawn and heat-set, is ordinarily used as the material of a base film for a magnetic recording tape. This film is roughly divided into a balanced type where the orientation and mechanical properties in the longitudinal direction are relatively similar to those in the transverse direction and a tensilized type where the orientation and mechanical properties are much higher in the longitudinal direction than in the transverse direction. A film of the balanced type is ordinarily prepared by drawing the film at a similar drawing ratio in both the longitudinal and transverse directions, and in the film of this type, the F-5 value is lower than 15 Kg/mm.sup.2, more ordinarily lower than 13 Kg/mm.sup.2, in both longitudinal and transverse directions. On the other hand, a tensilized film is prepared by drawing the film at the total drawing ratio in the longitudinal direction at a level much higher than the drawing ratio in the transverse direction, and the F-5 value is ordinarily larger than 15 Kg/mm.sup.2 and in the range of 17 to 20 Kg/mm.sup.2 in many cases.
Films of both these types have merits and demerits in the stiffness, dimensional stability, preparation easiness and cost, and one of these two types is appropriately selected according to the intended use.
Properties required for a base film for magnetic recording tape are the fineness and uniformity of protrusions on the film surface which are necessary for ensuring the uniformity and the smoothness of a magnetic material coating layer to be formed thereon, and the running property and abrasion resistance capable of resisting repeated use. These surface protrusions may ordinarily be formed by including protrusion-forming particles of an inorganic substance or a polymerinsoluble substance such as a catalyst residue (hereinafter, for brevity, referred to as "internal particles") into the film. However, the running property and abrasion resistance are properties contrary to the fineness and uniformity of the surface projections (hereinafter referred to as "surface characteristics"). In the film of the balanced type, the surface protrusions are ordinarily dependent upon the size and number of the included particles. For example, if a great number of large particles are included in the film, the running property and abrasion resistance can be improved, but the surface protrusions become coarse and fatal defects, such as omission of recording, i.e., increase in dropout, reduction of the output power and increase of the noise, are brought about in the resulting magnetic recording tape. On the other hand, if fine particles are included, fine surface protrusions are formed, and reduction of the output power and occurrence of the dropout phenomenon can be prevented, but the slip characteristic is degraded, resulting in reduction of the running property and abrasion resistance.